The invention relates to microwave frequency variable attenuators and more particularly to a monolithic microwave integrated circuit (MMIC) variable attenuator incorporated into a microstrip transmission line system.
The use of a microstripline waveguide formed as part of an MMIC in a microwave system that includes a microwave amplifier is known in the art. The nominal gain of such an amplifier in a microwave system can vary unit-to-unit by 10 dB or more due to production variables. In practice, a microwave attenuator follows the amplifier to reduce the amplifier output to a reference level. Known MMIC attenuators typically use PIN diodes or field effect transistors (FETs) often in a ".pi." or "T" configuration.
Prior art MMIC microstripline attenuators are deficient in several respects. They generally require two opposite polarity power supplies for operation and require that two independent DC control voltages be changed simultaneously to vary attenuation while maintaining a substantially constant attenuator input and output impedance typically 50 .OMEGA.. Adjustment of the two control voltages is critical and often very non-linear. Known MMIC microstripline attenuators cannot maintain attenuation flatness within 0.5 dB over a bandwidth of 2-20 GHz. Further, such attenuators exhibit limited dynamic range often less than 10 dB at maximum attenuation and cannot readily handle power in excess of about 0.25 W.